do we want to study 11th and 12th portions completely or shall we buy another book like guides or something? which are the chapters needed from our 11th and 12 th for this exam?

Hello Aspirant,

Since you have not mentioned the name of examination for which you are seeking guidance releated to books so it is little bit difficult to guide you with an accurate information but if we go through your profile than it shows that you are an engineering aspirant so you might be interested in JEE-Main related assistance, if not then please correct me in the comment section below.

So yes JEE-Main syllabus is the combined syllabus of class 11th and 12th so you must go through NCERT textbooks of all subjects and apart from those you should have some reference books for practice which I am enlisting down  :-

@Physics

Concepts of Physics-H. C. Verma

Fundamentals of Physics-Halliday, Resnick and Walker

Problems in General Physics-I. E. Irodov

University Physics-Freedman and Young

Understanding JEE physics series-DC Pandey

Problem In Physics-SS Krotov

@Mathematics

Higher Algebra-Hall and Knight

Coordinate Geometry-S.L. Loney

Differential and Integral Calculus-N. Piskunov

Calculus and Analytic Geometry-Thomas and Finney

IIT Mathematics for JEE Main & Advanced-M.L. Khanna

Course in Mathematics for IIT-JEE-Tata McGraw Hill publication

Problems Plus in IIT Mathematics-A Das Gupta

Trigonometry-S L Loney

@Chemistry

Modern Approach to Chemical Calculations-RC Mukherjee

Organic Chemistry-Solomons and Fryhle

Concise Inorganic-J. D. Lee

Organic Chemistry-Morrison and Boyd

University Chemistry-Freedman and Young

Physical Chemistry-N Awasthi

Arihant's Practice Book Chemistry for JEE Main And Advanced-R.K Gupta

You can also check out the JEE-Main knockout package for preparation of JEE-Main from the link given below :-

https://learn.careers360.com/knockout-jee-main-april/?product=27&icn=knockout-jee-main-2021-january&ici=

I hope this information helps you.

Good Luck!!